1. Field of the Invention
The present invention relates to a polishing method, and more particularly to a polishing method of polishing a workpiece such as a semiconductor wafer with a fixed abrasive.
2. Description of the Related Art
As semiconductor devices become more highly integrated in recent years, circuit interconnections have become finer and distance between those circuit interconnections becomes smaller. In case of photolithography which can form interconnections that are at most 0.5 xcexcm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because a depth of focus of an optical system is relatively small. A polishing apparatus for performing chemical mechanical polishing (CMP) has been used for planarizing a semiconductor wafer.
This type of chemical mechanical polishing (CMP) apparatus comprises a polishing table having a polishing pad attached thereon, and a top ring for holding a workpiece, to be polished, such as a semiconductor wafer. The workpiece is disposed between the polishing pad and the top ring and pressed against the polishing pad under a certain pressure by the top ring while the polishing table and the top ring are being rotated. The workpiece is polished to a flat mirror finish while a polishing liquid (slurry) is being supplied onto the polishing pad.
When the aforementioned chemical mechanical polishing process is continuously performed, polishing particles or polishing wastes are attached to the polishing pad, resulting in a change in properties of the polishing pad and a deterioration in polishing performance. Therefore, if an identical polishing pad is repeatedly used for polishing semiconductor wafers, problems such as lowered polishing rate and uneven polishing are caused. In order to overcome such problems, conditioning called dressing is performed before, after or during polishing of a semiconductor wafer to regenerate the polishing pad.
In a chemical mechanical polishing process using a polishing liquid as described above, a workpiece is polished while a polishing liquid containing a large amount of abrasive particles is being supplied onto a relatively soft polishing pad. Therefore, a problem of pattern dependence arises. Pattern dependence means that gentle irregularities are formed on a surface of a semiconductor wafer after a polishing process due to irregularities on the surface of the semiconductor wafer that existed before the polishing process, thus making it difficult to planarize the surface of the semiconductor wafer to a completely flat surface. Specifically, a polishing rate is higher in an area where irregularities have small pitches (a density of irregularities is large) and is lower in an area where irregularities have large pitches (a density of irregularities is small), and existence of areas of the higher polishing rate and areas of the lower polishing rate causes gentle irregularities to be formed on the surface of the semiconductor wafer. Further, during the polishing process using the polishing pad, since not only convexities but also concavities of the irregularities on the surface of semiconductor wafer are polished, it is difficult to stop the polishing process when the convexities of the irregularities are polished to a flat surface.
It has also been practiced to polish a semiconductor wafer with use of a fixed abrasive (grindstone) which comprises abrasive particles of cerium oxide (CeO2) or the like fixed by a binder such as phenolic resin. A polishing process utilizing the fixed abrasive is advantageous in that polishing material, i.e., the fixed abrasive, is harder than a polishing pad used in a conventional CMP process, and tends to polish convexities of the irregularities more than concavities thereof, for thereby achieving a higher absolute level of planarity. Depending on composition of the fixed abrasive, the fixed abrasive provides a self-stop function which considerably lowers a polishing rate and practically stops a polishing process when the convexities of the irregularities are polished to a flat surface. The polishing process utilizing the fixed abrasive is also advantageous in that environmental load can be reduced because of no use of a suspension liquid (slurry) containing a large amount of abrasive particles.
However, when a dressing process is performed on the aforementioned fixed abrasive, massive particles (masses of polishing particles) tend to be produced on a surface of the fixed abrasive. The massive particles may enter a boundary between the semiconductor wafer and the fixed abrasive to cause scratches to be produced on a surface of the semiconductor wafer. After the semiconductor wafer is polished with the fixed abrasive, abrasive particles contained in the fixed abrasive are attached to the surface of the semiconductor wafer. Therefore, it is necessary to prevent the semiconductor wafer from being contaminated by the abrasive particles.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a polishing method which can effectively remove massive abrasive particles produced on a surface of a fixed abrasive, by performing a dressing process, to prevent scratches from being produced on a surface of a workpiece, and can remove abrasive particles attached to a surface of a workpiece after a polishing process to prevent the workpiece from being contaminated.
According to a first aspect of the present invention, there is provided a method comprising: polishing a workpiece by pressing the workpiece against a fixed abrasive and bringing the workpiece into sliding contact with the fixed abrasive; dressing a surface of the fixed abrasive so as to generate free abrasive particles thereon; and ejecting (or atomizing) a liquid or a gas, composed of a mixture of liquid or inert gas and pure water or chemical liquid, onto the surface of the fixed abrasive during or after the dressing of the surface of the fixed abrasive.
As described above, when the dressing process is performed on the fixed abrasive, massive particles (masses of polishing particles) tend to be produced on the surface of the fixed abrasive. According to the present invention, atomization is performed on the surface of the fixed abrasive during the dressing process or immediately after the dressing process. Therefore, even if massive particles, which cause scratches on a surface of the wafer, are produced on the surface of the fixed abrasive by the dressing process, the atomization can remove the massive particles from the surface of the fixed abrasive to prevent the workpiece from being scratched.
It is free fine abrasive particles present on the surface of the fixed abrasive that contribute to a polishing process of the workpiece. These free fine abrasive particles are unlikely to be removed by atomization. Therefore, while massive particles are removed as described above, the free fine abrasive particles which contribute to a polishing rate are not removed, and the polishing rate is not affected by atomization. However, if pressure of an inert gas is higher than 0.5 MPa, then the fine abrasive particles are likely to be removed to thereby lower the polishing rate. Therefore, it is desirable that a flow rate of the liquid is within a range of from 200 to 5000 cm3/min, and pressure of the inert gas is within a range of from 0.05 to 0.5 MPa. More preferably, the flow rate of the liquid is about 1000 cm3/min, and the pressure of the inert gas is about 0.15 MPa.
Such atomization may be performed in either case of an in-situ dressing process in which a dressing process is performed during a polishing process of a workpiece, or an ex-situ dressing process in which a dressing process is performed when the workpiece is not polished. The atomization should preferably be performed during the dressing process. Particularly, in the case of the in-situ dressing process, it is necessary to perform the atomization during the dressing process.
The liquid should preferably be ejected toward an outer peripheral edge of the fixed abrasive. When the liquid is ejected toward the outer peripheral edge of the fixed abrasive, the aforementioned massive particles can efficiently be removed from the surface of the fixed abrasive. When gas is ejected onto the surface of the fixed abrasive for removing the massive particles, the liquid is supplied together with the gas onto the surface of the fixed abrasive. The gas is ejected onto the liquid supplied to the surface of the fixed abrasive to thereby promote removal of foreign matter from the surface of the fixed abrasive.
According to a second aspect of the present invention, there is provided a method comprising: polishing a workpiece by pressing the workpiece against a fixed abrasive and bringing the workpiece into sliding contact with the fixed abrasive while dressing a surface of the fixed abrasive; and continuously polishing the workpiece while not dressing the surface of the fixed abrasive.
From a viewpoint of wear resistance, the dressing process should preferably be performed with a dressing tool (diamond dresser) having particulates such as diamond particles electrodeposited thereon.
As described above, if massive particles are produced on the surface of the fixed abrasive by performance of the dressing process and enter the boundary between the workpiece and the fixed abrasive, then the massive particles are crushed to cause scratches to be produced on a surface of a workpiece. According to the present invention, the workpiece is polished while dressing a surface of the fixed abrasive, and then is polished while not dressing the surface of the fixed abrasive. Therefore, even if massive particles are produced on the surface of the fixed abrasive by performance of the dressing process, the massive particles are crushed or removed from the surface of the fixed abrasive during the polishing process, so that scratches are not newly produced on the surface of the workpiece. Specifically, although scratches may be produced on a surface of a workpiece during an initial stage of a polishing process, the scratches can gradually be shallowed by continuously polishing the workpiece and can finally be eliminated. Further, although scratches are continuously produced on a surface of a workpiece during an in-situ dressing process, these scratches can be eliminated by stopping the dressing process and continuing a polishing process.
In a dressing process before a polishing process, i.e., an ex-situ dressing process, when atomization is performed together with the above method, scratches can be prevented more effectively. It is desirable to supply pure water (DIW) during the dressing process, and ultra pure water or a chemical liquid containing no abrasive particles as a polishing liquid during the polishing process, onto the surface of the fixed abrasive.
If surface pressure during a dressing process is high, then the number of massive particles (masses of polishing particles) produced becomes large, and larger massive particles are likely to be produced. As a result, the number of scratches on the workpiece is increased, and depths of the scratches become greater. Since such massive particles hardly contribute to improvement of a polishing rate, it is desirable that the number of massive particles be small. Therefore, surface pressure during the dressing process should preferably be set to be as small as possible, 9.8 kPa or lower, for example. Further, it is desirable that surface pressure of the workpiece, when the workpiece is being polished while not dressing the surface of the fixed abrasive, be set to be smaller than that when the workpiece is being polished while dressing the surface of the fixed abrasive.
According to a third aspect of the present invention, there is provided a method comprising: polishing a workpiece by pressing the workpiece against a fixed abrasive and bringing the workpiece into sliding contact with the fixed abrasive; and then water-polishing the workpiece while supplying pure water to the fixed abrasive, wherein surface pressure of the workpiece during the water-polishing is set to be smaller than that during the polishing using the fixed abrasive.
During the water-polishing process, abrasive particles attached to the surface of the workpiece can be cleaned and removed from a surface of the workpiece. The water-polishing process should preferably be performed for 5 seconds or longer.
A rotational speed of the fixed abrasive (polishing table) during the polishing process is usually 30 revolutions per minute or smaller. The rotational speed of the polishing table should preferably be increased to 50 revolutions per minute or higher to enhance an effect of cleaning and removing abrasive particles attached to the surface of the workpiece.
When a fixed abrasive is used for polishing a workpiece, the workpiece may be polished even during a water-polishing process. In order to prevent such a drawback, it is necessary to reduce surface pressure of the workpiece as small as possible. Although surface pressure of the workpiece during the polishing process using the fixed abrasive is usually 29.4 kPa or higher, surface pressure of the workpiece during the water-polishing process should preferably be set to be smaller than that during the polishing process using the fixed abrasive. Specifically, the surface pressure of the workpiece during the water-polishing process should preferably be reduced to 29.4 kPa or lower, more preferably 20 kPa or lower.
Water-polishing should preferably comprise supplying pure water at a flow rate larger than a flow rate of a polishing liquid supplied during polishing using a fixed abrasive.
When a workpiece is separated or removed from a surface of the fixed abrasive, a portion of the workpiece is exposed beyond an outer peripheral edge of the fixed abrasive so as not to leave the wafer on the surface of the fixed abrasive, which is called an overhanging process. However, if rotational speed of the polishing table is high, then the workpiece cannot stably be held at a overhanging position by a top ring so as to cause scratches or unevenly polished portions to be produced on the workpiece by the outer peripheral edge of the fixed abrasive. Therefore, when the workpiece is lifted from the fixed abrasive (polishing table) during the overhanging process, rotational speed of the polishing table should preferably be reduced to 10 revolutions per minute or lower. The workpiece should preferably be removed directly from the surface of the fixed abrasive without the overhanging process, in which a portion of the workpiece is exposed beyond the outer peripheral edge of the fixed abrasive.
According to a fourth aspect of the present invention, there is provided a method comprising: polishing a workpiece by pressing the workpiece against a fixed abrasive and bringing the workpiece into sliding contact with the fixed abrasive; and then cleaning (or buff cleaning) the workpiece by pressing the workpiece against a soft cleaning surface and supplying a liquid containing no abrasive particles to the cleaning surface.
When the workpiece is polished with the fixed abrasive, abrasive particles contained in the fixed abrasive are likely to be attached to a surface of the workpiece immediately after the polishing process. Particularly, abrasive particles of ceria are easily attached to a surface of a silicon oxide film. According to the present invention, after the polishing process, the workpiece is pressed against a soft cleaning surface, and a liquid containing no abrasive particles is supplied to the cleaning surface. With a cleaning process, the abrasive particles attached to the surface of the workpiece can be removed from the surface of the workpiece. Soft surface means a surface having a large modulus of compression elasticity.
In this case, pure water may be supplied to the cleaning surface. It is more effective to supply an alkali liquid, preferably an alkali liquid having a pH of 9 or larger because surface potential (zeta potential) of ceria and an oxide film becomes negative in an alkali region, and the ceria and the oxide film become likely to become dissociated from each other by repulsion. When the alkali liquid contains tetramethylammonium hydroxide (TMAH), the ceria and the oxide film are more likely to become dissociated from each other, so that an effect of removing the abrasive particles can be enhanced.
According to a fifth aspect of the present invention, there is provided a method comprising: polishing a workpiece by pressing the workpiece against a fixed abrasive and bringing the workpiece into sliding contact with the fixed abrasive; and cleaning (or DHF cleaning) a surface of the workpiece with dilute hydrogen fluoride after the polishing of the workpiece.
When the workpiece is polished with the fixed abrasive, abrasive particles contained in the fixed abrasive are likely to be attached to a surface of the workpiece after the polishing process. In order to remove the abrasive particles attached to the surface of the workpiece, a DHF cleaning process may be performed instead of a buff cleaning process. When dilute hydrogen fluoride is added to a surface of a silicon oxide film, the oxide film is slightly dissolved. For example, when a DHF liquid of 0.5% is added to the surface of the silicon oxide film for about 30 seconds, the oxide film is dissolved by a thickness of about 50 xc3x85. Thus, the oxide film is dissolved and removed together with abrasive particles attached to the surface of the oxide film. In this case, it is desirable to use a DHF liquid having a concentration of 0.1% or higher.
When scrubbing of a workpiece by a roll sponge, a pencil-type sponge, or a brush is accompanied with this DHF cleaning, the abrasive particles can be removed more effectively. It is desirable to perform the DHF cleaning process immediately after the polishing process of the workpiece. However, the DHF cleaning process may be performed on a workpiece which has already been dried.
In a polishing method of pressing a workpiece against a polishing surface and bringing the workpiece into sliding contact with the polishing surface to polish the workpiece, a polishing liquid containing abrasive particles may be supplied to the polishing surface after the polishing process of the workpiece to perform a final polishing. In this case, the polishing surface may comprise a fixed abrasive or a polishing pad other than the fixed abrasive (for example, IC1000 or POLITEX made by Rodel Corp).
According to a sixth aspect of the present invention, there is provided a method comprising: polishing a workpiece by pressing the workpiece against a first polishing tool having a diameter and bringing the workpiece into sliding contact with the first polishing tool, while supplying a chemical liquid containing no abrasive particles to the first polishing tool; and further polishing the workpiece by subjecting the workpiece to a second polishing tool having a diameter larger than the diameter of the first polishing tool while supplying a polishing liquid containing abrasive particles to the second polishing tool.
According to a seventh aspect of the present invention, there is provided a method comprising: polishing a workpiece with a fixed abrasive while supplying a chemical liquid to a surface of the fixed abrasive; and simultaneously ejecting at least one of a liquid containing the chemical liquid, and a fluid composed of a mixture of inert gas and the chemical liquid, onto the surface of the fixed abrasive. In this case, the chemical liquid should preferably comprise an anionic surface-active agent. The workpiece should preferably comprise a semiconductor wafer on which a pattern of STI is formed.
According to an eighth aspect of the present invention, there is provided a polishing apparatus comprising: a holding device for holding a workpiece; a polishing table having a fixed abrasive thereon, the fixed abrasive including abrasive particles and a binder; a dressing device for generating free abrasive particles from the fixed abrasive; an ejection nozzle for ejecting a fluid onto a surface of the fixed abrasive to remove massive particles, which adversely affect a polishing process, from the surface of the fixed abrasive; a controller for adjusting a relative speed between the polishing table and the holding device; and a controller for adjusting a pressing force produced between the polishing table and the holding device.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.